Radio receiver



Mard'n 24, 1936. MCLENNAN RADIO RECEIVER Filed Jan. 27, 1934 INVENTORMiles A.MCLenna,n

BY I V '0 L l V mu ATTORNEY amkk 053% l NJ fir Patented Mar. 24, 1936UNITED STATES PATENT OF ICE RADIO RECEIVER Miles A. McLennan, .Riverton,N. J., assignor to Radio Corporation of America, a 'corporation ofDelaware My invention relates to radio receivers and more particularlyto receivers of the superheterodyne type provided with automatic volumec0ntrol devices.

During the reception of radio signals undesirable distortion is oftennoticed. One cause of 'Such distortion is the phenomenon known asselective fading occasioned by interference, at the point of reception,between the so-called ground wave and the sky-wave reflected from theheaviside layer. Such interference may reduce the amplitude of thecarrier with respect to the side bands and the received signal is thenanalogous to that radiated by a transmitter which is over modulated.

It is, accordingly, an object of my invention to provide a radioreceiver that shall automatically compensate the effects of selectivefading.

Another object of my invention is to provide a radio receiver that shallautomatically minimize distortion during the reception of over-modulatedcarrier waves.

Radio receivers having automatic volume-control devices ordinarily havetwo objectionable features, namely, the difficulty of exactly tuningin adesired signal unless a tuning meter or the like is provided and,further, the increase in sensitivity during intervals when the tuning isbeing changed from the frequency of one signal to that of another. Suchincrease in sensitivity gives rise to back-ground noise which isextremely unpleasant.

Another object of my invention, therefore, is to provide aradio-receiver of the automatic volumecontrol type that shall besemi-automatic in tuning; that is to say, a receiver that requiresmanual adjustment to only approximately the proper frequency after whichfurther adjustment is accomplished automatically.

A still further object of my invention is to provide, in a radioreceiver of the type described, automatic means for back-ground noisesuppression during the operation of tuning.

In a preferred embodiment of my invention, I achieve the foregoingobjects and other objects ancillary thereto by providing a localoscillator, adapted to operate at an intermediate frequency, forcompensating over-modulation in an incoming signal, together with meanswhereby energy derived from the oscillator is utilized in conjunctionwith the incoming signal for the purpose of blocking one or more stagesof the receiver until such time as a desired signal is actually tunedin. Furthermore, still in accordance with my invention, I so dispose thevarious elements of thereitself, however, both as to its organizationand its method of operation, together with additional objects andadvantages thereof, will .best be understood from the followingdescriptionof a specific embodiment when read in connection with theaccompanying drawing, in which;

Figure 1 is a diagrammatic view of a portion of a radio receiverembodying my invention, and

Fig. 2 is a diagram to which reference will be made in explaining theoperation of the receiver illustrated in Fig. 1.

Referring to Fig. 1 of the drawing, an illustrative receiver constructedaccording to my invention may most easily'be explained by considering itas divided into three distinct though mutually cooperative portions,namely, a substantially conventional multi-stage superheterodynereceiver, an'automatic volume control device and a group of devicesfunctioning to provide both background suppression and additional energyto compensate for carrier fading or over-modulation. Specifically, thereceiver portion may'comprise one or more radio-frequency amplifyingstages, exemplified in the drawing by the thermionic device I, a localoscillator 3, a first detector 5, a pluralityof intermediate frequencyam-, plifiers l, 9, and II, a second detector [3 and an audio frequencyamplifier (not shown).

As is customary in modern radio receivers, all plate and bias potentialsmay be derived from a single source of rectified alternating current.This source is exemplified, in the drawing,- -by a bleeder resistori5,to points on which a plurality of leads ll, l9, 2|, 23, and 25 extendfrom the various thermionic devices in the receiver.

To avoid the necessity of providing anti-oscillation devices inconnection with the amplifier tubes, and to obtain satisfactory gaintherein, such tubes are preferably of the equipotentialcathodescreen-grid type. Connections from the screen grids to the bleederresistor have not been shown nor have connections for supplyingpotential to the cathode heaters, since their existence in an actualreceiver is obvious to those .skilled' in the art.

For the purpose of obtaining automatic volume control, the control gridof the radio frequency amplifier I, the grid of the first intermediatefrequency amplifier l and that of the second intermediate frequencyamplifier 9 are returned to points in the receiver that become morenegative with respect to their cathodes with increase in signalamplitude. These points are found on a resistor 21 included in the diodeoutput circuit of a thermionic AVC of the diode-pentode typecommercially known as radiotron 2B7,the cathode 29 of which is connectedthrough a resistor 3| and the lead I! to the bleeder resistor l at apoint more negative than the ground connection 33 thereto. Since thecathodes of the controlled tubes l and 9 are directly connected toground, that portion of the bleeder resistor between its groundconnection 33 and the connection l1 thereto of the AVG tube cathodeprovides a fixed bias on the said controlled tubes in the absence ofadditional potential across the diode output resistor 21.

In addition to the cathode, the AVG tube includes a control grid 35, asuppressor grid 31, an anode 39 and a diode plate 4 l. Thediode plate ofthe tube is connected back to the cathode thereof through a circuit 43,tuned to the intermediate frequency, and the diode resistor 21 which hasa value of approximately one megohm, and the anode 39 is connected tothe bleeder resistor through a tuned circuit 45, coupled to the firstmentioned tuned circuit 43, and the lead 23.

The incoming signal is applied to the AVG tube from the input circuit ofthe second IF tube over a connection, including a stopping condenser 41,extending between the control grids of the several tubes. In the absenceof incoming signal potential across the input circuit of the AVG tube,corresponding to the condition of the receiver while tuning from onestation to the next, the bias on the grids of the first RF tube and thefirst and second IF tubes is determined solely by the potential dropalong the bleeder resistor 15 between the ground connection 33 and theconnection thereto of the AVG tube cathode.

As soon, however, as an incoming signal is impressed upon the inputcircuit of the AVG tube, it is amplified therein, appears in the outputcircuit 45 connected to the anode thereof and is fed back to the diodeplate through the coup-led circuit 43. The signal, accordingly, isrectified in the diode portion of the tube and an average unidirectionalpotential corresponding to the amplitude of the intermediate frequencyis developed across the output resistor 21. With such an arrangement,the unidirectional current flowing in the resistor, during reception ofa signal, is a1 ways proportional to the amplitude of the carrier andthe end of the resistor connected to the diode plate becomesincreasingly negative as the carrier amplitude increases, thus reducingthe gain in the RF amplifier and the first and second IF amplifiers. Thenet result is that the gain in the system is reduced as signal amplitudeincreases and vice versa, thus causing substantially constant signalpotentials to be impressed upon the input circuit of the third IF tube.

A radio receiver such as I have thus far described has no means forsuppressing background noise during the tuning operation, whensensitivity is maximum, and the automatic volume control feature rendersit very difiicult to the tuning operation but it has been my observationthat meters have very little practical utility.

Therefore, in accordance with my invention, I provide a further group ofinstrumentalities that conjointly function to give back-ground noisesuppression, semi-automatic tuning and deficient carrier-wave, orovermodulaticn, compensation.

Specifically, the said instrurnentalities include an oscillator 5|,tuned to the intermediate frequency, and a thermionic tube 53,hereinafter designated the suppressor tube, which is preferably of theequipotential-cathode diode-triode type commercially known as radiotron55. It is to be clearly understood, however, that my invention is notlimited to the specific tube chosen for purposes of illustration.

The suppressor tube is provided with a bias resistor 55, the cathode endof which is connected to the cathode of the third IF tube l I and theother end of which is connected to ground. Since the grid of the tube His also connected to ground, thesaid self bias resistor therefore isincluded in the input circuit of the said tube.

Under normal conditions, when no sigal is being received, a fairly heavycurrent flows through the self bias resistor 55 and through a glow tube5'! connected between the anode of the suppressor tube and the lead 25which extends to a point on the bleeder resistor at a positivepotentialwith respect to ground. The self bias resistor, preferably, has a valueof approximately 509 ohms and the potential developed across it issufiicient to bias the third IF tu'be H to cutoff during the tuningoperation, thus giving noise suppression.

For the purpose of removing the bias as soon as a signal is tuned in, itis necessary to provide signal-responsive means for controlling the biason the grid of the suppressor tube. For this purpose, I connect thediode plate in the suppressor-tube to the cathode thereof through acircuit including a coil 59 coupled to the output circuit of the IFoscillator 5|, a sharply tuned circuit 9| which is coupled to the inputcircuit of the third IF amplifier l l, and a resistor 63 having value ofapproximately one megohm. By reason of the coupling between the diodecircuit and the IF oscillator, a certain amount of rectified currentflows in the resistor 63 and also a certain amount flows therein, as asignal is being tuned in, by reason of the coupling between the diodecircuit and the input circuit of the third IF amplifier tuber As amatter of fact, however, there is very little change in the currentflowing in the resistor and, consequently, very little change in thepotential developed across it until the intermediate frequency and theIF oscillator frequency are in phase. Therefore, since the grid of thesuppressor tube, as will be seen from the draw-. ing, is connected tothe negative terminal of the resistor 63, there will be but very slightchange in its potential until the receiver is so tuned that theintermediate frequency delivered from the IF amplifier 9 differs fromthe normal frequency of the oscillator 5| by not more than approximatelyKC. This condition is exemplified by tube. Through proper choice of theconstants of the several circuits, depending upon the tubes used, thislocking in can be caused to take place atnot more than 500 cycles awayfrom the mean intermediate frequencyand the oscillator will stay lockedin over a range of substantially one kilocycle.

It will be noted from an inspection of the drawing that I have chosen toillustrate the secnd detector l3 as a thermionic tube of the coplanargrid type, one grid being coupled to the output circuit of thethirdintermediate frequency amplifier and the other grid being connecteddirectly to the grid of the IF oscillator. By reaison of this latterconnection, the IF oscillator supplies to the second detector sufficientintermediate frequency potential to offset the efiect of selectivefading, over modulation at the transmitter and the distortion producedin the detector by an over modulated signal.

This introduction of the intermediate frequency would cause veryannoying interference effects at each side of the tuned-in point were itnot for the fact that as soon as the third intermediate frequencyamplifier ll opens up the IF oscillator locks in with the then effectiveintermediate frequency. These simultaneous actions prevent any beatnotes from being generated through interaction between oscillations fromthe IF oscillator and the intermediate frequency appearing in the outputcircuit of the second IF amplifier 9.

From a consideration of the foregoing, it will be apparent that I haveprovided an improved background noise suppressing circuit whereby it isimpossible to mis-tune the receiver. A receiver constructed according tomy invention permits a station to be audibly tuned in over only a veryrestricted range of the tuning dial, which range may, through properchoice of constants of the system, be fixed at any desired value from 10to 500 cycles on each side of the mean intermediate frequency. Such anarrangement prevents distortion resulting from the mis-alignment oftuning controls, which is a common occurrence with present day receiversof types including automatic volume control devices.

My improved receiver also gives an apparent increase in selectivity overreceivers in use prior to my invention and provides a still furtherreduction in between-station noise usually heard on such receivers,especially those having automatic volume control devices.

Furthermore, by reason of the introduction of the intermediate frequencyinto the second detector, my improved receiver exhibits a veryremarkable reduction of distortion caused by carrier cancellation orselective fading, which distortion at times totally destroys theintelligibility of received speech and introduces effects into musicwhich are very objectionable.

In addition, the glow lamp included in the anode circuit of thesuppressor tube begins to blink on and off as the correct tuning pointis approached and finally is extinguished completely at the instant thestation is exactly tuned in. As shown in the drawing, the glow lamp maybe utilized to illuminate the usual tuning dial if desired.

Many other advantages of my invention will be apparent to those skilledin the art to which it pertains, as well as many obvious modificationsthereof. The specific circuit described has been chosen merely forpurposes of illustration and my invention is not to be limited exceptinsofar as is necessitated by the prior art and-by the spirit of theappended claims.

I claim as my invention:

1. Ina multi-stage radio receiver, means for converting an incomingsignal to an intermediate frequency signal, means for biasing one of thestages to substantially the cut-off potential, an oscillator, and meansconjointly responsive topo tentials representing said intermediatefrequency signal and to potentials derived from said oscillatorforrendering the biasing means inoperative-Z- 2. In a multi-stage radioreceiver, means for converting an incoming signal to anin'teime'diatefrequency signal, means for biasing one of the" stages to substantiallythe cut-off potential during the operation of tuning, an oscillator,-area: tifier, connections for impressing amplified" oscil lationsrepresentative of said intermediate frequency signal and potentials fromsaid oscillator on the rectifier, and connections whereby theunidirectional current from the rectifier is ut'ilizedftir the purposeof rendering the'biasing means in operative.

3. In a tunable radio receiver of the superheterodyne type, means forconverting an incoming modulated carrier to an intermediate fre' quencysignal, means including an oscillator tuned to said intermediatefrequency for compensating selective fading of said incomingmodulated'car rier and means for effectively restrainingthe a'c-' tionof the compensating means as the receiver is tuned towards an incomingsignal untilthe 're'-' ceiver is tuned to a frequency differing fromthefrequency of said signal by a relatively small number of cycles.

4. In a tunable radio receiver of the multi-stage type, means forconverting an incoming signal to an intermediate frequency signal, anoscillator, a rectifier, connections for impressing said intermediatefrequency signals upon said rectifier, connections for impressingpotentials from said oscillator upon said rectifier, means for biasingone of the stages .101 the cut-off potential during the absence of anincoming signal, and means, responsive to the output of said rectifierfor rendering the biasing means inoperative.

5. The invention set forth in claim 4, characterized in that the biasingmeans is constituted by a multi-electrode thermionic tube provided witha self-bias resistor and further characterized in that the resistor isconnected in the input circuit of the stage to which the bias isapplied.

6. The invention set forth in claim 1, characterized in that the biasingmeans is constituted by a thermionic device provided with an outputcircuit including a resistor and. having a control electrode, andfurther characterized in that the means conjointly responsive topotentials representing said intermediate frequency signal and topotentials derived from the oscillator is constituted by a rectifierhaving an output circuit including a resistor the negative end of whichis connected to the control electrode.

"1. The invention set forth in claim 1 characterized in that another oneof the stages includes a detector and further characterized in that aconnection extends between the oscillator and the said detector.

8. The invention set forth in claim 1 characterized in that the biasingmeans is constituted by a thermionic tube having an output circuit andfurther characterized in that a current-responsive device is included inthe said output circuit.

9. Ina radio receiver, tuning means, manually controllable means foradjusting the tuning means to syntony with a desired incomingradiosignal, means for silencing the receiver during the tuningoperation until approximate syntony with the desired signal is attained,and signal-responsive means including an oscillator for abruptlyrendering the silencing means inoperative when approximate syntony isattained and for maintaining the frequency of said oscillator unchangedand said silencing means inoperative during a further adjustment of thetuning means over a limited range, whereby the action of automatictuning is simulated.

10. In a superheterodyne radio receiver, tuning means including afrequency converting oscillator, manually controllable means foradjusting the tuning means to syntony with a desired incoming radiosignal, means for silencing the receiver during the tuning operationuntil approximate syntony with the desired signal is attained, a localoscillator and means conjointly responsive to the intermediate frequencysignal and to potentials derived from said local oscillator for abruptlyrendering the silencing means inoperative when approximate syntony isattained and for maintaining the said silencing means inoperative duringa further adjustment of the tuning means over a limited range, wherebythe action of auto.-

matic tuning is simulated.

resenting the intermediate frequency and potentials derived from thelocal oscillator and from which is derived an unidirectional potential,which, through suitable connections, is utilized to control the flow ofcurrent in the space current device.

12. The invention set forth in claim 10, characterized in that thecircuit constants of the local oscillator are such as to tune itnormally to the intermediate frequency, and additionally characterizedin that the said oscillator is coupled to one of the intermediatefrequency amplifying stages, the coefiicient of coupling beingsufficient to cause the said local oscillator to lock in with theintermediate frequency and to remain locked in therewith over a limitedrange of adjustment of the tuning means.

13. The invention set forth in claim 10, characterized in that means areprovided for impressing upon the second detector potentials derived fromthe local oscillator.

14. The invention according to claim 10, characterized in that thesecond detector has a control grid upon which the intermediate frequencysignal is impressed and a second grid, and further characterized in thatmeans are provided for impressing upon said second grid potentialsderived from said local oscillator.

MILES A. MCLENNAN.

DISCLAIMER 2,035,176.Miles A. McLermcm, Rivercon, N. J. RADIO RECEIVER.Patent dated March 24, 1936. Disclaimer filed December 15, 1937, by theassignee, Radio Corporation of America. Hereby enters this disclaimer toclaims 3 and 10 in said specification.

[Oflicial Gazette January 4, 1988.]

